In the field of data communications, modems are used to convey information from one location to another. digital subscriber line (DSL) technology now enables DSL devices, such as DSL modems, to communicate large amounts of digital data. Typically in a communications environment, Plain Old Telephone Service (POTS) type devices (such as telephones, facsimile machines, and dial modems) conveying conventional telecommunications signals are connected to the same subscriber wire pair as DSL devices at the user's location, which is typically remote from the telephone company's central office location, via a wire pair provided by the local telephone company.
Because passband DSL signals, such as asymmetric digital subscriber line (ADSL) and rate adaptive digital subscriber line (RADSL) modem signals, typically occupy only the frequency band above the audio band, these DSL signals have traditionally been isolated from all POTS type devices (such as telephones or dial modems) by a splitter or filter system installed at the user (remote) location. Such a splitter is typically known in the field of telephony communications as a POTS splitter. The POTS splitter typically serves two purposes: (1) it attenuates the DSL signals so that they do not significantly appear at the input of the POTS devices, and (2) it attenuates the POTS signals so that they do not significantly appear at the input of the DSL devices. In particular, the POTS filter attempts to attenuate DSL signals appearing at the input of the POTS devices in the audio band to an inaudible level, and also attempts to attenuate DSL signals above the audio band to a level low enough so that distortion inside the POTS type devices does not adversely affect their performance.
Although POTS splitters have been effective, such splitters are undesirable for many applications because of installation and cost issues. Recognizing the undesirable attributes of POTS splitters, efforts to eliminate them have begun in the DSL industry and standards bodies. These attempts call for "splitter-less-DSL" systems wherein the DSL devices and the POTS devices are all directly connected to the two-wire communications channel without the use of a POTS splitter.
In splitter-less operation, it is necessary for the DSL device to: (1) filter its output signal to ensure that signals in the audio band are below an audible level, and (2) reduce its output (transmit) signal power to a level that does not cause adverse distortion in the POTS devices. The reduction in output signal power is a crucial aspect of splitter-less operation. Unfortunately, such reduction may dramatically reduce data performance, in some cases to an unacceptable level. Due to line losses, reduction in the output signal power level also reduces the reach of the subscriber loop.
To facilitate production of a splitter-less DSL device, such as a DSL modem, that can be installed in a variety of systems without field adjustment, the DSL device transmit level must be pre-set to the lowest level commensurate with the worst distortion situation expected for the potential universe of attached POTS devices. As noted above, such a reduced output level may be undesirable in many installations. Although many installations might tolerate a higher output level, this level could only be determined by trial and error for each installation, which is a cumbersome process and impractical for mass deployment of DSL splitter-less devices for consumer applications.
Due to the problems involved in using many DSL devices in splitter-less operation, it has been proposed that phone filters (also known as distributed POTS splitters) be placed at the interface to each item of POTS type equipment at the remote location. A phone filter is a bi-directional lowpass filter that attenuates as much of the DSL modem signal as practical and prevents it from appearing at the interface to the POTS type devices, such as telephones and dial modems. A phone filter can markedly improve splitter-less operation by permitting the transmit power level of the DSL device to be increased above the level that would produce distortion in splitter-less operation. However, phone filters also have cost and installation drawbacks and they are less suitable for some types of DSL operation than a system utilizing a POTS splitter. Thus, it is desirable to eliminate phone filters where they are not required, for example on a particular POTS device that is immune to distortion.
Therefore, there is a need in the industry for a method and apparatus for determining at a DSL device whether the distorting effects of the DSL device's transmit signal warrant the incorporation of a phone filter at the interface to each POTS device in the system. There is also a need to be able to make such determinations at both the user (remote) end and at the central site end of the two-wire communications channel.